[TenTec] OmniVI Plus Over Drive Amp

[Dr. Gerald N. Johnson]

On Sun, 2008-06-22 at 12:47 -1000, Ken Brown wrote:
> 
> > What you need are TWO Birds. Running them cascaded you will discover
> > that the input Z of a Bird feeding a matched load isn't necessarily a
> > matched load. E.g. the Bird changes the impedance seen. To minimize
> > that, Bird in their manuals and catalog, recommends that a cable be used
> > with the Bird to make the total added line length any integral multiple
> > of a half wave in coax. So what you are seeing might not be faulty cable
> > construction, but rather just a poor length of cable. These impedance
> > changing effects are greater the more sensitive the slug because the
> > more sensitive slugs are more tightly coupled to the main transmission
> > line.
> >   
> >
> The input Z of a Bird 43 feeding a matched load is also a matched load, 
> as long as we are talking about a 50 ohm impedance system. The input Z 
> of a Bird 43 feeding an unmatched load is an unmatched load, and that 
> load is not the same Z as without the Bird in line. Just like any added 
> length of feed line when there is significant SWR on the line, the Z 
> changes along the line.

Try two birds in cascade before you are so sure.

73, Jerry, K0CQ
> 
> It is true that Bird recommends using a specific cable length, depending 
> on the frequency you are using the instrument on, which "When added to 
> Bird 43 Thruline to Equal 1/2 Wavelength". Their chart for determining 
> the length of cable to use covers zero to 500 MHz, but only has data 
> above about 110 MHz where the length to be used is 35 inches. The idea 
> is that, when adding or removing a 1/2 wavelength of line, the SWR 
> conditions, and what the source (transmitter or amplifier) "sees" will 
> be the same either with or without the meter and jumper cable in line. 
> If the SWR is pretty good, that is very little reflected power, then it 
> doesn't much matter. And for the HF bands the five inches of line length 
> added by the Bird 43 is pretty insignificant.
> 
> I keep a set of cables with my Bird 43, for use on VHF and UHF. I have 
> never made or used a special cable for ten meters or any of the HF 
> bands, and I don't think it is necessary.
> 
> DE N6KB